Methods and systems for constraint of multiple spine segments

ABSTRACT

Methods, apparatus and systems for constraining spinous processes to elastically limit flexion of two or more adjacent spinal segments rely on placing a tether structure over at least three adjacent vertebral bodies or two adjacent vertebral bodies and the sacrum. The tether structures may be continuous, for example in the form of a continuous loop, or may be discontinuous, for example in the form of a loop or elongate element having at least two anchor structures for securing in bone.

The present application is a continuation-in-part of U.S. PatentApplication No. PCT/US2007/081822 (Attorney Docket No. 026398-000140PC)filed Oct. 18, 2007, which claims priority to U.S. Provisional PatentApplication No. 60/862,085 (Attorney Docket No. 026398-0001000US) filedOct. 19, 2006. The present application also is a non-provisional of, andclaims the benefit of U.S. Provisional Patent Application No. 61/158,892(Attorney Docket No. 026398-001300US) filed Mar. 10, 2009. The entirecontents of each of the above applications is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to medical methods andapparatus. More particularly, the present invention relates to methodsand devices for restricting spinal flexion in patients having back painor other spinal conditions.

A major source of chronic low back pain is discogenic pain, also knownas internal disc disruption. Patients suffering from discogenic paintend to be young, otherwise healthy individuals who present with painlocalized to the back. Discogenic pain usually occurs at the discslocated at the L4-L5 or L5-S1 junctions of the spine (FIG. 1). Paintends to be exacerbated when patients put their lumbar spines intoflexion (i.e. by sitting or bending forward) and relieved when they puttheir lumbar spines into extension (i.e. arching backwards). Discogenicpain can be quite disabling, and for some patients, can dramaticallyaffect their ability to work and otherwise enjoy their lives.

This pain experienced by patients with discogenic low back pain can bethought of as flexion instability, and is related to flexion instabilitythat is manifested in other conditions. The most prevalent of these isspondylolisthesis, a spinal condition in which abnormal segmentaltranslation is exacerbated by segmental flexion. The device describedhere should as such also be useful for these other spinal disordersassociated with segmental flexion, for which the prevention or controlof spinal segmental flexion is desired.

Current treatment alternatives for patients diagnosed with chronicdiscogenic pain are quite limited. Many patients follow a conservativetreatment path, such as physical therapy, massage, anti-inflammatory andanalgesic medications, muscle relaxants, and epidural steroidinjections, but typically continue to suffer with a significant degreeof pain. Other patients elect to undergo spinal fusion surgery, whichcommonly requires discectomy (removal of the disk) together with fusionof adjacent vertebra. Fusion is not usually recommended for discogenicpain because it is irreversible, costly, associated with high morbidity,and of questionable effectiveness. Despite its drawbacks, however,spinal fusion for discogenic pain remains common due to the lack ofviable alternatives.

Recently, a less invasive and potentially more effective treatment fordiscogenic pain has been proposed. A spinal implant has been designedwhich inhibits spinal flexion while allowing substantially unrestrictedspinal extension. The implant is placed over one or more adjacent pairsof spinal processes and provides an elastic restraint to the spreadingapart of the spinal processes which occurs during flexion. Such devicesand methods for their use are described in U.S. Patent Publication No.2005/0216017A1, published on Sep. 29, 2005, (now U.S. Pat. No.7,458,981) and having common inventors with the present application. Theentire contents of U.S. Patent Publication No. 2005/0216017 A1 areincorporated herein by reference.

As illustrated in FIG. 2, an implant 10 as described in the '017publication, typically comprises an upper strap component 12 and a lowerstrap component 14 joined by a pair of compliance members 16. The upperstrap 12 is shown disposed over the top of the spinous process SP4 of L4while the lower strap 14 is shown extending over the bottom of thespinous process SP5 of L5. The compliance member 16 will typicallyinclude an internal element, such as a spring or rubber block, which isattached to the straps 12 and 14 in such a way that the straps may be“elastically” or “compliantly” pulled apart as the spinous processes SP4and SP5 move apart during flexion. In this way, the implant provides anelastic tension on the spinal processes which provides a force thatresists flexion without substantially limiting extension of the segment.The force increases as the processes move further apart. Usually, thestraps themselves will be essentially non-compliant so that the degreeof elasticity or compliance may be controlled and provided solely by thecompliance members 16.

Although providing significant benefits, the system illustrated in FIG.2 is intended to treat only a single spinal segment between a pair ofadjacent vertebral bodies. In some patients, it would be desirable totreat two or more successive spinal segments.

For these reasons, it would be desirable to provide improved spinalimplants, implant systems, and methods for their use for limitingflexion in two or more successive spinal segments. It would beparticularly desirable if the implants, systems, and methods permittedthe spinous processes of three or more adjacent vertebral bodies, or twoadjacent vertebral bodies and the sacrum, to be elastically coupledusing a single implant structure which can constrain multiple adjacentspinal features. At least some of these objectives will be met by theinventions described hereinbelow.

2. Description of the Background Art

US 2005/0216017A1 has been described above. Other patents and publishedapplications of interest include: U.S. Pat. Nos. 4,966,600; 5,011,494;5,092,866; 5,116,340; 5,282,863; 5,395,374; 5,415,658; 5,415,661;5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,609,634;5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133;5,964,769; 5,989,256; 6,053,921; 6,312,431; 6,364,883; 6,378,289;6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091;6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729;6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245;6,761,720; 6,835,205; Published U.S. Patent Application Nos. US2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US2004/0117017; US 2004/0127989; US 2004/0172132; US 2005/0033435; US2005/0049708; US 2006/0069447; Published PCT Application Nos. WO01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO03/045262 A1; WO 2004/052246 A1; WO 2004/073532 A1; and PublishedForeign Application Nos. EP 0322334 A1; and FR 2 681 525 A1.

BRIEF SUMMARY OF THE INVENTION

The present invention provides spinal implants, implant systems, andmethods for constraining spinous processes to elastically limit flexionof two or more adjacent spinal segments. As used herein, the phrase“spinal segment” is synonymous with the phrase “functional spinal unit(FSU)” and intended to mean the smallest physiological motion unit ofthe spine that exhibits biomechanical characteristics similar to thoseof the entire spine. A spinal segment or FSU consists of two adjacentvertebrae, the intervertebral disc and all adjoining ligaments betweenthem and excludes other connecting tissues such as muscles. Thethree-joint complex that results is sometimes referred to as the“articular triad.” Another term for the FSU is spinal motion segment.These definitions are taken from White A A, Panjabi M M. (1990),Clinical Biomechanics of the Spine, Philadelphia, J B Lippincott. Themethods comprise placing a tether structure over the spinous processesof at least three adjacent vertebral bodies, or over the spinousprocesses of two adjacent vertebral bodies and a sacrum, wherein thestructure elastically couples the at least two non-adjacent spinousprocesses or one spinous process and a non-adjacent sacrum. The spinousprocesses and optionally a sacrum can be interconnected and elasticallycoupled in a variety of ways.

In a first exemplary pattern, the tether structure elastically couplesan upper spinous process to a lower spinous process, or to the sacrum,with at least one intermediate spinous process being free from coupling.In an alternative pattern, the tether structure elastically couples anupper spinous process and a lower spinous process or sacrum, as well asthe at least one intermediate spinous process. The spinous processes andoptionally the sacrum may be elastically coupled by a single contiguoustether structure, or in other embodiments may be elastically connectedby two or more contiguous tether structures. In the case of two or morecontiguous tether structures, the tether structures may further beinterconnected, coupled, or linked in order to provide desired elasticrestraint characteristics. The spinous processes being connected willtypically be in the lumbar region, most typically being at the lowerlevels of the lumbar, and even more particularly being at L3, L4, L5 andthe sacrum. In most instances, the spinous processes, and optionally thesacrum, are elastically coupled to inhibit flexion with the spacesbetween the adjacent vertebral bodies being free from structure whichwould substantially limit or inhibit extension of the spinal segmentsbeing treated. A first portion and a second portion of the tetherstructure may extend between the upper spinous process and the lowerspinous process or the sacrum. The first and second portions of thetether structure may be disposed symmetrically on opposite sides of thespinous processes and they also may be parallel to one another.

In another aspect of the present invention, a spinal implant comprises acontiguous tether structure adapted to circumscribe at least twonon-adjacent spinous processes, or in other instances, to an anchorlocation on the sacrum and one non-adjacent spinous process. At least aportion of the tether structure will provide an elastic resistance toelongation in response to an elongation force which results from flexionof the spinal segments between the non-adjacent spinous processes and/orbetween the one non-adjacent spinous process and the sacrum. The tetherstructure limits flexion therebetween without substantially limitingextension therebetween. A first portion and a second portion of thetether structure may extend between the non-adjacent spinous processesor between the one non-adjacent spinous process and the sacrum. Thefirst and second portions of the tether structure may be disposedsymmetrically on opposite sides of the spinous processes and they alsomay be parallel to one another. Often, the implant will include at leasttwo compliance members positioned as part of the tether structure suchthat they will lie symmetrically on opposite sides of the spinousprocesses when implanted. In still other embodiments, the contiguoustether structures will include at least four such compliance members.The compliance members will typically be coupled to non-compliant and/orcable components of the tether structure so that it is the compliancemembers which provide most or all of the compliance or elasticity in theimplants. Exemplary compliance structures are illustrated in copendingU.S. Patent Publication No. 2005/02161017 A1 (now U.S. Pat. No.7,458,981).

In some embodiments, the contiguous tether structure will be continuousso that the structure forms a loop which may be placed over thenon-adjacent spinous processes. Such continuous “loop” tether structureswill usually be maintained on the spinous processes by friction andinterference fit, but in some cases could be modified to permit furtherattachment by stapling, welding, gluing, suturing, or the like. In otherembodiments, the contiguous tether structure will be discontinuous andwill have two ends which are adapted for anchoring for direct attachmentto the bone. Such discontinuous tether structures will be suitable foranchoring in the sacrum.

In a third aspect of the present invention, systems comprising a spinalimplant as generally described above further include at least oneadditional contiguous tether structure. The additional tether structurewill usually be adapted to circumscribe two adjacent or non-adjacentspinous processes or a sacrum. The additional contiguous tetherstructures may be continuous so that they can be looped over the spinousprocesses, or in other instances may be discontinuous and have two endsadapted for anchoring directly in the bone. The additional contiguoustether structure may be interconnected with the primary tether structurebut will frequently be formed separately.

In still another aspect of the present invention, a method forconstraining spinous processes to elastically limit flexion of two ormore adjacent spinal segments comprises placing a first tether structureover a superior spinous process and an inferior spinous process of afirst spinal segment. The first tether structure elastically couples thesuperior spinous process and the inferior spinous process so as to limitflexion therebetween without substantially limiting extension thereof. Afirst portion of the first tether structure extends between the superiorspinous process and the inferior spinous process of the first spinalsegment, and a second portion of the first tether structure extendsbetween the superior spinous process and the inferior spinous process ofthe first spinal segment. The first and the second portions are disposedsymmetrically on opposite sides of the spinous processes, and they aresubstantially parallel to one another. The method also comprises placinga second tether structure over a superior spinous process and aninferior spinous process or a sacrum of a second spinal segment. Thesecond tether structure elastically couples the superior spinous processand the inferior spinous process or the sacrum of the second spinalsegment so as to limit flexion therebetween without substantiallylimiting extension thereof. A first portion of the second tetherstructure extends between the superior spinous process and the inferiorspinous process or the sacrum of the second spinal segment, and a secondportion of the second tether structure extends between the superiorspinous process and the inferior spinous process or the sacrum of thesecond spinal segment. The first and the second portions are disposedsymmetrically on opposite sides of the spinous processes, and they aresubstantially parallel to one another. The first spinal segment isadjacent and superior to the second spinal segment, and one of the firstor second tether structures is positioned anteriorly relative to theother tether structure.

In some embodiments, the tether structure may be disposed around a firstsurface of the a spinous process and a second tether structure may bepositioned around a second surface of the spinous process, opposite thefirst surface. The two tethers may be positioned on the spinous processsuch that one tether is anteriorly disposed on the spinous processrelative to the other tether structure.

In other embodiments, the tether structure may have a first compliancemember with a first elasticity and a second compliance member with asecond elasticity different than the first elasticity. The tetherstructure may also comprise a first pair of compliance members and asecond pair of compliance members. Each of the first pair may have afirst elasticity and each of the second pair may have a secondelasticity. The first elasticity may be the same or different than thesecond elasticity. The first pair of compliance members may be superiorto the second pair of compliance members.

In preferred embodiments, the tether structure inhibits or limitsflexion of a spinal segment without substantially limiting extensiontherebetween. Thus, in some embodiments, the tether structure may havean elastic stiffness in compression below 3 N/mm and in otherembodiments the elastic stiffness in compression may be below 0.5 N/mm.

In still other embodiments, the tether structure may be positioned overan upper spinous process, a lower spinous process and an intermediatespinous process disposed therebetween. The tether structure may comprisea first loop encircling the lower spinous process and the intermediatespinous process so as to substantially prevent flexion therebetween, andthe tether structure may also comprise a second loop superior to thefirst loop. The second loop may have one or more compliance members andmay be disposed over the upper spinous process and coupled with thefirst loop so as to provide a force resistant to flexion of a superiorspinal segment relative to the inferior spinal segment.

These and other embodiments are described in further detail in thefollowing description related to the appended drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the lumbar region of thespine including the spinous processes (SP), facet joints (FJ), lamina(L), transverse processes (TP), and sacrum (S).

FIG. 2 illustrates a spinal implant of the type described in US2005/0216017A1.

FIG. 3 illustrates a contiguous tether structure constructed inaccordance with the principles of the present invention and adapted forplacement over three adjacent spinous processes.

FIG. 4 illustrates a contiguous tether structure similar to that shownin FIG. 3 which further includes four symmetrically placed compliancestructures.

FIG. 5 illustrates a contiguous tether structure constructed inaccordance with the principles of the present invention which is adaptedfor placement over three adjacent spinous processes and which furtherincludes an intermediate loop segment for engaging the intermediatespinous process.

FIG. 6 illustrates a contiguous tether structure similar to that shownin FIG. 5, where the intermediate loop structure is adjustably attachedto the main tether structure.

FIG. 7 illustrates a contiguous tether structure constructed inaccordance with the principles of the present invention and adapted forplacement over four adjacent spinous processes including twointermediate loop structures and six symmetrically placed compliancemembers.

FIG. 8 illustrates yet another contiguous tether structure constructedin accordance with the principles of the present invention comprisingtwo loop segments joined together by connectors adjacent an intermediatespinous process.

FIG. 8A illustrates another embodiment of a tether structure constructedin accordance with the principles of the present invention comprisingtwo loop segments.

FIG. 9 illustrates a contiguous tether structure constructed inaccordance with the principles of the present invention and having adiscontinuous structure with two ends adapted for anchoring in thesacrum.

FIG. 10 illustrates a system constructed in accordance with theprinciples of the present invention and including two contiguous tetherstructures which may be used simultaneously.

FIG. 11 illustrates a sagittal view of tethers applied to adjacentspinous processes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods, devices, and systems forconstraining the flexion of two or more adjacent spinal segments byelastically restraining two or more spinous processes or at least onespinous process and an anchor region on a sacrum. Such restraint isachieved using a tether structure which spans at least three spinousprocesses or a pair of spinous processes and the sacrum (morespecifically, the spinous processes on L4 and L5 as well as an anchorregion on the sacrum). The tethers used will typically be in the form ofa contiguous tether structure. By “contiguous” it means that the tethermay comprise one or more elongate component(s), such as strap(s),cable(s), ribbon(s), or the like, which may be constructed or modifiedto provide for a desired elastic coupling of one or more spinousprocesses and optionally an anchor location on the sacrum.Alternatively, the “contiguous” tether structures may comprise aplurality of components, such as the straps, bands, cables, or the like,as mentioned above, together with compliance structures which providefor the desired elastic coupling. In the latter case, the straps, etc.,will typically be non-compliant, effecting little or no elongation inresponse to tension, while the compliance members will provide thedesired level of elastically coupling. Combinations of compliantelongate components and separate compliance members will also bepossible.

The contiguous tether structures may be continuous or discontinuous. The“continuous” contiguous tether structures will typically be formed intoa loop so that the loop may be placed over a pair of spinous processes,typically non-adjacent spinous processes separated by at least oneintermediate spinous process. The “discontinuous” contiguous tetherstructures, in contrast, will have at least two free ends adapted withanchor structures for anchoring to bone, typically to anchor regions ona sacrum.

Referring now to FIG. 3, a first exemplary continuous tether structure20 is shown circumscribing the spinous processes SP1-SP3 on the L1-L3vertebral bodies. The tether structure 20 may be a simple band, strap,or cable which is formed into a continuous loop, where at least aportion of the structure provides a desired elasticity to inhibitflexion of the spinal segments between L1 and L2 and L2 and L3 in acontrolled manner. Elasticity may be provided through use of anelastomeric material, inclusion of spring-like or elastic regions in anotherwise inelastic or non-compliant structure, or the like.

Referring now to FIG. 4, a second exemplary continuous tether structure24 is similar to tether 20, except that it is provided with separatecompliance structures 26 a-26 d arranged symmetrically on opposite sidesof the “ridge” of spinous processes. The tether structure 24 is shownplaced on the spinous processes SP2-SP4 on vertebral bodies L2-L4, itwill be appreciated that tether structures 20 and 24 could be placed onany three contiguous spinous processes, typically in the lumbar region.Various combinations of elasticities may be used amongst the fourcompliance members 26 a-26 d. For example, all four compliance membersmay have the same elasticity. Alternatively, all four compliance membersmay have an elasticity different from one another. In some embodiments,the two superior compliance members 26 a, 26 b may have a firstelasticity and the two inferior compliance members 26 c, 26 d may have asecond elasticity different than the first. This allows the resistanceto flexion to be varied at different levels of the spinal segment. Instill other embodiments, at one motion segment level, the elasticity ofa left compliance member 26 a may be different than the right compliancemember 26 b. One of skill in the art will appreciate that anycombination of elasticities may be employed in a tether structure havingmultiple compliance members. This applies to any of the embodimentsdisclosed herein having multiple compliance members.

The continuous tether structures of the present invention may be formedin multiple interconnected loops, as shown, for example, in FIGS. 5-8.The multiple loops will usually include an outer or peripheral loopwhich encircles or otherwise engages at least three or more adjacentspinous processes. One or more inner loops may also be provided toengage or encircle one, two, or possibly more of “intermediate” spinousprocesses within the group which is being restrained.

For example, in FIG. 5, a continuous tether structure 30 includes anouter loop 32 which encircles three adjacent spinous processes, shown asSP3-SP5 on vertebral bodies L3-L5. An inner loop 34 is provided whichencircles only SP4 and SP5. The upper portions of the two loops 32 and34 are both connected in compliance members 36 a and 36 b. Thecompliance members may be configured to apply a generally equal elastictensioning to the upper loop portions as the spinal segments undergoflexion. Alternatively, the compliance members 36 a and 36 b could beconfigured to provide different elastic tensioning forces to the uppersegments of loops 32 and 34, respectively.

Continuous tether structure 40, as shown in FIG. 6, also comprises anouter loop 42 (shown to encircle SP3-SP5) and an inner loop 44 (shown toencircle SP4 and SP5 only), similar to the tether structure 30 of FIG.5. An upper loop portion 46, however, is shown attached to slidingattachment members 48 a and 48 b, which attachment members allow theupper loop structure 46 to be tightened or “cinched” over the top ofSP4. The tether structure 40 is also shown with four symmetricallyplaced compliance members 50 a-50 d, but it will be appreciated that thetether structure could include only two or even no compliance members,while retaining the adjustably placed upper loop structure 46. Asdiscussed above, any combination of elasticities may be used amongst thecompliance members.

A more complex continuous tether structure 60 including one externalloop and two internal loops is illustrated in FIG. 7. The external loopis configured to circumscribe four adjacent spinous processes (SP2-SP4)while the first internal loop defined by loop segment 64 extends overSP3 and a second internal loop segment 66 extends over SP4. Sixcompliance members 68 a-68 f are provided symmetrically on oppositesides of the spinous processes, and the ends of the first upper loopsegment 64 are connected to compliance members 68 c and 68 d,respectively, while the ends of the second loop segment 66 are connectedto compliance members 68 e and 68 f, respectively. It will beappreciated that the use of six compliance members and the twointermediate loop segments allows the tension on each of the spinousprocesses to be independently adjusted to some extent. The overallcompliance and elastic force applied to the spinal segments, however,will depend on the cumulative value of the elastic forces provided byall of the compliance members. Thus, elasticity may be varied amongstthe compliance members, as previously discussed above.

A multiple loop tether structure 70 having a more simple configurationis shown in FIG. 8. An upper loop 72 is adapted to circumscribe a pairof adjacent spinous processes (shown as SP2 and SP3) while a lower loopis adapted to circumscribe an overlapping pair of spinous processes(shown as SP3 and SP4). The two loops are joined by connector components76 a and 76 b which may be simple clips or crimps to hold the loops 64and 66 together (in which case the loops would likely be elastic orpartially elastic to allow for controlled flexion of the spinalsegments) or could be compliance members which provide for controlled,elastic movement of the upper loop 72 relative to the lower loop 74. Inthe latter case, the loops would likely be non-compliant.

FIG. 8A illustrates another embodiment similar to that of FIG. 8, excepthere the tether structure is coupled to two adjacent spinous processesat a first level of the spinal segment and another portion of the tetherstructure having compliance members is then coupled to a superiorspinous process so that flexion is restricted in the suprajacentsegment. This may be used, for example, when the spinal segment isfused. In FIG. 8A, a first part of the tether structure consists of atether 102 circumscribing two adjacent spinous processes SP3-SP4. Thetether 102 is disposed around a superior surface of a superior spinousprocess SP3 and also around an inferior surface of an inferior spinousprocess SP4. A fusion according to methods known in the art has beenperformed to fuse L3-L4 together at, or across the level designated by Fand therefore tether 102 will often be substantially inelastic in orderto prevent flexion between L3-L4 thereby facilitating the fusion F,although some micromotion is still permitted. The tether structure alsohas a second tether 104 disposed around a superior surface of a superiorspinous process SP2 superior to the fused region. The ends of the secondtether 104 are coupled with the first part of the tether structure 102,or in alternative embodiments, the ends of the second tether 104 arecontinuous forming a closed loop and thus are disposed under theinferior surface of SP3. Compliance members 106 a, 106 b provide a forceresistant to flexion of the L2-L3 motion segment supradjacent to thefused region F. This helps to more evenly distribute and possibly lessenloading applied to the fused region, to the level superior to the fusedregion, and to tethers 102 and 104. The tether structure may also helpto reduce excessive motion. Additional details on the use of a tetherstructure concomitantly with fusion are disclosed in U.S. ProvisionalPatent Application Nos. 61/158,892 (Attorney Docket No. 026398-001300US)and 61/158,886 (Attorney Docket No. 026398-001400US), both filed on Mar.10, 2009, and both of which the entire contents are incorporated hereinby reference.

The contiguous tether structures of the present invention will notalways have a continuous structure. As shown in FIGS. 9 and 10, thetether structures may also have a discontinuous geometry including atleast two ends adapted to anchor to bone, typically to a surface of thesacrum which generally lacks structure for attaching the lower end of aloop. As shown in FIG. 9, an exemplary discontinuous tether structure 80comprises a U-shaped tether or band structure including compliancemembers 82 a and 82 b. A pair of anchor structures 84 a and 84 b areprovided on two ends of the tether structure 80 and are adapted to beanchored into the face of the sacrum S, as illustrated. In this way, thetether structure 80 can provide for controlled elastic restraint of thespinal segments between SP4 and SP5 and between SP5 and the sacrum.Additional details on sacral attachment may be found in U.S. ProvisionalPatent Application No. 61/149,224 (Attorney Docket No. 026398-001200US),filed Feb. 2, 2009, and U.S. patent application Ser. No. 11/827,980(Attorney Docket No. 026398-000120US), filed Jul. 13, 2007. The entirecontents of each of these applications is incorporated herein byreference.

FIG. 10 illustrates a system including a tether structure 24, generallyas described with reference to FIG. 4 above, and a second tetherstructure 90 which is similar to tether structure 80, except that it isadapted only to extend around a single spinous process (SP5) and to beanchored into the sacrum S. Attachment may be provided in a variety ofways as described in copending application Ser. Nos. 11/827,980 and61/149,224, both previously incorporated herein by reference. The secondtether structure may be attached using a dowel implanted in the sacrum,using alar screws, using superior articular facet screws, using toggleanchors (T-tags) placed in holes formed in a superior articular facet ofS1, using hooks attached to the dorsal S1 foramen, or the like. Thetether structure 24 and tether structure 90 could be deployed withoutany interconnection, as generally shown in FIG. 10. Often, however, itmight be desirable to interconnect the tether structures at theircrossover points 92 a, 92 b, generally adjacent to the two sides of SP5.The attachment could be accomplished using a crimp structure (not shown)or by otherwise tying, welding, or fusing the tether structurestogether.

FIG. 11 illustrates a sagittal view of a spinal segment and shows thepositioning of tethers around the spinous processes. In FIG. 11, a firsttether structure 120 having two compliance members 122 (only one visiblein this view) is disposed over a superior surface of a superior spinousprocess SSP and also is disposed under an inferior surface of anintermediate spinous process MSP. A second tether structure 124 havingtwo compliance members 126 (only one visible in this view) is disposedover a superior surface of the intermediate spinous process MSP andunder an inferior surface of the inferior spinous process ISP. Inembodiments where multiple tethers are coupled to a spinous process,such as in SP5 in FIG. 10, the first tether may be coupled to thespinous process and it is often advanced in the anterior direction inorder to allow room for the second tether which will be slightlyposterior to the first tether. FIG. 11 shows the first tether structure120 anterior to the second tether structure 124 on the intermediatespinous process MSP. While this embodiment shows a slight gap betweenthe two tethers, the two tethers may also be pushed against one anotherso there is no gap, or in some embodiments the two tethers may slightlyoverlap one another.

In each of the embodiments disclosed herein, the tether structure limitsflexion of a spinal segment. Additionally, because the tether structureis flexible and has a low elastic stiffness in compression, it does notsubstantially limit extension of the spinal segment. Any of theembodiments may utilize tether structures that have an elastic stiffnessin compression below 3 Newtons per millimeter (N/mm). In someembodiments the elastic stiffness in compression may be below 0.5 N/mm.

It will be appreciated that numerous other combinations of continuoustether structures and discontinuous tether structures could be providedin order to effect the controlled application of elastic restraint onadjacent spinal segments in the lumbar region of the spine. Thus, theexamples set forth above are not meant to be limiting on the breadth ofthe invention as set forth in the following claims.

1. A method for constraining spinous processes to elastically limitflexion of two or more adjacent spinal segments, said method comprising:placing a tether structure over spinous processes of at least threeadjacent vertebral bodies, or of two adjacent vertebral bodies and asacrum, wherein the structure elastically couples an upper spinousprocess and a lower spinous process or sacrum so as to limit flexiontherebetween without substantially limiting extension thereof, andwherein a first portion of the tether structure extends between theupper spinous process and the lower spinous process or the sacrum, and asecond portion of the tether structure extends between the upper spinousprocess and the lower spinous process or the sacrum, the first and thesecond portions disposed symmetrically on opposite sides of the spinousprocesses, and substantially parallel to one another.
 2. A method as inclaim 1, wherein the tether structure elastically couples an upperspinous process and a lower spinous process or sacrum with at least oneintermediate spinous process free from coupling.
 3. A method as in claim1, wherein the tether structure elastically couples an upper spinousprocess, a lower spinous process or sacrum, and at least oneintermediate spinous process.
 4. A method as in claim 3, wherein upperspinous process, intermediate spinous process, and lower spinous processor sacrum are coupled by a single contiguous tether structure.
 5. Amethod as in claim 3, wherein the upper spinous process, intermediatespinous process, and lower spinous process or sacrum are coupled by atleast two contiguous tether structures.
 6. A method as in claim 1,wherein the tether structure is disposed around a first surface of aspinous process, and the method further comprises positioning a secondtether structure around a second surface of the spinous process, whereinthe second surface is opposite the first surface, and wherein the secondtether positioned on the spinous process such that one tether isanteriorly disposed relative to the other tether structure.
 7. A methodas in claim 1, wherein a lower one of the vertebral bodies is selectedfrom the group consisting of L4, L5, and the sacrum.
 8. A method as inclaim 1, wherein spaces between the adjacent vertebral bodies are freefrom structure which would inhibit extension.
 9. A method as in claim 1,wherein the tether structure comprises one or more band elements inseries with one or more compliance members.
 10. A method as in claim 1,wherein the tether structure comprises at least two compliance members,further comprising positioning the compliance members symmetrically tolie on opposite sides of the spinous processes.
 11. A method as in claim1, wherein the tether structure comprises at least four compliancemembers, further comprising positioning pairs of the compliance memberssymmetrically on opposite sides of the spinous processes.
 12. A methodas in claim 1, wherein the tether structure comprises a first compliancemember having a first elasticity and a second compliance member having asecond elasticity, the first compliance member superior to the secondcompliance member, and wherein the first elasticity is different thanthe second elasticity.
 13. A method as in claim 1, wherein the tetherstructure comprises a first pair of compliance members and a second pairof compliance members, wherein the first pair of compliance members eachhave a first elasticity and the second pair of compliance members eachhave a second elasticity, the first elasticity being different than thesecond elasticity, and wherein the first pair of compliance members aresuperior to the second pair of compliance members.
 14. A method as inclaim 1, wherein the tether structure is disposed over the upper spinousprocess, the lower spinous process and an intermediate spinous processdisposed therebetween, and wherein the tether structure comprises afirst loop encircling the lower spinous process and the intermediatespinous process so as to substantially prevent flexion therebetween, andwherein the tether structure comprises a second loop superior to thefirst loop, the second loop having one or more compliance members anddisposed over the upper spinous process and coupled with the first loopso as to provide a force resistant to flexion of a superior spinalsegment relative to the inferior spinal segment.
 15. A method as inclaim 1, wherein the tether structure provides an elastic stiffness incompression below 3 N/mm.
 16. A method as in claim 15, wherein theelastic stiffness in compression is below 0.5 N/mm.
 17. A spinal implantcomprising a contiguous tether structure adapted to circumscribe atleast two non-adjacent spinous processes or an anchor location on asacrum and one non-adjacent spinous process, wherein at least a portionof the tether structure provides an elastic resistance to elongation inresponse to an elongation force which results from flexion of the spinalsegments between the non-adjacent spinous processes or the onenon-adjacent spinous process and the sacrum, and wherein the tetherstructure limits flexion therebetween without substantially limitingextension therebetween, and wherein a first portion of the tetherstructure extends between the non-adjacent spinous processes or betweenthe one non-adjacent spinous process and the sacrum, and a secondportion of the tether structure extends between the non-adjacent spinousprocesses or between the one non-adjacent spinous process and thesacrum, the first and the second portions disposed symmetrically onopposite sides of the spinous processes, and substantially parallel toone another.
 18. A spinal implant as in claim 17, further comprising atleast two compliance members positioned symmetrically to lie on oppositesides of the spinous processes.
 19. A spinal implant as in claim 17,further comprising at least four compliance members positionedsymmetrically to lie on opposite sides of the spinous process.
 20. Aspinal implant as in claim 17, further comprising a first compliancemember having a first elasticity and a second compliance member having asecond elasticity, the first compliance member superior to the secondcompliance member, and wherein the first elasticity is different thanthe second elasticity.
 21. A spinal implant as in claim 17, furthercomprising a first pair of compliance members and a second pair ofcompliance members, wherein the first pair of compliance members eachhave a first elasticity and the second pair of compliance members eachhave a second elasticity, the first elasticity being different than thesecond elasticity, and wherein the first pair of compliance members aresuperior to the second pair of compliance members.
 22. A spinal implantas in claim 17, wherein the contiguous tether structure is continuous toloop over said non-adjacent spinous processes.
 23. A spinal implant asin claim 17, wherein the contiguous tether structure is discontinuousand has two ends, each end having an anchor for attachment to bone. 24.A spinal implant as in claim 17, wherein the tether structure isdisposed over an upper spinous process, a lower spinous process and anintermediate spinous process disposed therebetween, and wherein thetether structure comprises a first loop encircling the lower spinousprocess and the intermediate spinous process so as to substantiallyprevent flexion therebetween, and wherein the tether structure comprisesa second loop superior to the first loop, the second loop having one ormore compliance members and disposed over the upper spinous process andcoupled with the first loop so as to provide a force resistant toflexion of a superior spinal segment relative to the inferior spinalsegment.
 25. A spinal implant as in claim 17, wherein the tetherstructure provides an elastic stiffness in compression below 3 N/mm. 26.A spinal implant as in claim 25, wherein the elastic stiffness incompression is below 0.5 N/mm.
 27. A system comprising a spinal implantas in claim 17, and at least one additional contiguous tether structureadapted to circumscribe two adjacent or non-adjacent spinous processesor a sacrum.
 28. A system as in claim 27, wherein the additionalcontiguous tether structure is continuous to loop over said non-adjacentspinous processes.
 29. A system as in claim 27, wherein the additionalcontiguous tether structure is discontinuous and has two ends, each endhaving an anchor for attachment to bone.
 30. A system as in claim 27,wherein the contiguous tether structure is disposed around a firstsurface of a spinous process, and the additional tether structure isdisposed around a second surface of the spinous process, wherein thesecond surface is opposite the first surface, and wherein the additionaltether structure is positioned on the spinous process such that onetether is anteriorly disposed relative to the other tether structure.31. A method for constraining spinous processes to elastically limitflexion of two or more adjacent spinal segments, said method comprising:placing a first tether structure over a superior spinous process and aninferior spinous process of a first spinal segment, wherein the firsttether structure elastically couples the superior spinous process andthe inferior spinous process so as to limit flexion therebetween withoutsubstantially limiting extension thereof, wherein a first portion of thefirst tether structure extends between the superior spinous process andthe inferior spinous process of the first spinal segment, and a secondportion of the first tether structure extends between the superiorspinous process and the inferior spinous process of the first spinalsegment, the first and the second portions disposed symmetrically onopposite sides of the spinous processes, and substantially parallel toone another; and placing a second tether structure over a superiorspinous process and an inferior spinous process or a sacrum of a secondspinal segment, wherein the second tether structure elastically couplesthe superior spinous process and the inferior spinous process or thesacrum of the second spinal segment so as to limit flexion therebetweenwithout substantially limiting extension thereof, wherein a firstportion of the second tether structure extends between the superiorspinous process and the inferior spinous process or the sacrum of thesecond spinal segment, and a second portion of the second tetherstructure extends between the superior spinous process and the inferiorspinous process or the sacrum of the second spinal segment, the firstand the second portions disposed symmetrically on opposite sides of thespinous processes, and substantially parallel to one another, andwherein the first spinal segment is adjacent and superior to the secondspinal segment, and wherein one of the first or second tether structuresis positioned anteriorly relative to the other tether structure.